Acetylenes removal from olefin streams for alkylation by dimethyl formamide absorption

ABSTRACT

Dimethyl formamide when used as an absorption solvent to remove acetylenes from olefin-containing gas streams acts as well as a promoter for the oxidative conversion of H 2  S, COS and/or CS 2 , also contained in the feed stream, to elemental sulfur. Thus both acetylenes and sulfur compounds are efficiently removed from such olefin-containing gas streams as coke oven gas. Further, the loss of dimethyl formamide entrained and vaporized by the deacetylenized gas stream leaving the dimethyl formamide absorber is substantially reduced or effectively eliminated by the injection of a stream of liquid selected from the group consisting of paraffinic hydrocarbons having 4-8 carbon atoms per molecule containing at least one tertiary carbon atom per molecule and aromatic hydrocarbons having 6-10 carbon atoms per molecule above the point of injection of the dimethyl formamide solvent stream to the absorption column. This liquid stream injection is particularly advantageous when the stream is also a reactant in a downstream alkylation process.

This invention relates to the hydrocarbon separation. In one aspect thisinvention relates to the removal of acetylenes from gas streamscontaining acetylenes and olefins.

In another aspect this invention relates to the desulfurization of gasstreams containing acetylenes, olefins and sulfur compounds.

BACKGROUND OF THE INVENTION

It is well known in the art that N,N-dimethyl formamide (DMF) is anefficient selective solvent to remove, e.g., acetylenes from gas streamscontaining such acetylenes in addition to olefins. One of the problemsoccurring in such an absorption process is that a small portion of thedimethyl formamide leaves the absorption zone overhead together with thedeacetylenized gas. It already has been proposed to remove entrainedselective solvent by contacting the absorber overhead gas streamraffinate with liquid ethylene. This procedure, generally applied to theremoval of acetylene (C₂ H₂) from ethylene (C₂ H₄), which can basicallybe characterized as refluxing the absorption column with liquefiedethylene overhead gas, is fairly complicated and expensive becauseethylene must be first liquefied and fed to the column with precisecontrol to prevent substantial loss in the rich DMF kettle product fromwhich the acetylenes are later stripped. If a separate contacting unitis utilized, as has been proposed in the art, the ethylene/dimethylformamide mixture must later be separated so that a plurality of unitsis then required, which adds to the overall cost of the process and mayrender it economically unattractive.

It would thus be desirable to have a process available by which theexpensive dimethyl formamide solvent leaving the dimethyl formamideabsorber overhead with the product gas is readily and relativelyinexpensively recovered.

Furthermore, it is known in the prior art that such commerciallyavailable gas streams as coke oven gas contain sulfur compounds such asH₂ S, COS and CS₂. These low-boiling sulfur compounds are veryundesirable and need to be removed to prepare suitable feedstock forprocesses utilizing such gases. Thus it would be desirable to have aprocess available by which the sulfur compounds in gases such as cokeoven gas can be readily removed as well.

THE INVENTION

It is thus one object of this invention to provide a new process forextracting or absorbing acetylenes from gas streams containingacetylenes and olefins.

Another object of this invention is to provide a process for theabsorption of acetylenes from gases containing acetylenes and olefinsessentially without removal of dimethyl formamide overhead with thedeacetylenized gas stream.

Furthermore, it is an object of this invention to provide a solventabsorption process utilizing dimethyl formamide as the solvent, and inwhich essentially no dimethyl formamide is lost.

Yet another object of this invention is to provide a process for thedesulfurization of coke oven gas containing oxidizable sulfur compoundssuch as H₂ S, COS, CS₂ and free oxygen.

Still a further object of this invention is to provide a process fordesulfurizing and deacetylenizing coke oven gas containing sulfurcompounds comprising H₂ S, COS, CS₂ and free oxygen, olefins andacetylenes.

These and other objects, advantages, details, features and embodimentsof the present invention will become apparent to those skilled in theart from the following detailed description of the invention, theexamples, the appended claims and the drawing which shows a schematicflow diagram of a deacetylenizing and desulfurizing unit for coke ovengas.

In accordance with one aspect of this invention, we have now found thata gas containing olefins, acetylenes, oxygen and one or more oxidizablesulfur compounds, e.g., selected from the group consisting of H₂ S, COS,and CS₂ can be both effectively deacetylenized and desulfurized bycontacting this gas with liquid N,N-dimethyl formamide.

The liquid N,N-dimethyl formamide absorbs or extracts the acetylenes,whereas the sulfur compounds are converted or oxidized to elementalsulfur. This absorption and reaction process preferably is carried outin the presence of a small quantity of water, e.g., 0.01 to 1.00 part byweight of water per 100 parts by weight of pure dimethyl formamide. Anabsorber kettle product stream, comprising a liquid phase of dimethylformamide with dissolved acetylenes, and a minor second phase of solidor liquid elemental sulfur (depending upon temperature) is formed. Thisrich solvent stream is later readily separated into an acetylene gasstream, a liquid dimethyl formamide solvent stream for recycle to thedimethyl formamide absorber and a liquid sulfur stream by heatstripping.

The liquid effluent from the dimethyl formamide absorber, in which theacetylenes are selectively dissolved in the liquid dimethyl formamidefrom a feed gas comprising acetylenes and olefins, can be passed to aflash tank wherein, by means of heat exchange and pressure reduction, agas stream can be flash-vaporized from the rich solvent comprisingprincipally olefins (butylenes, propylene and ethylene) with someacetylene and methyl acetylene included. This gas stream can berecompressed and injected as stripping gas into the lower portion of theabsorption column. The partially stripped or denuded solvent is passedto the DMF stripper or deacetylenizer, wherein the acetylenes aredistilled off and removed as an overhead gas, and the stripped, liquiddimethyl formamide stream, in which a minor amount of a separate liquidsulfur phase is dispersed, is removed from the bottom of the stripper.The two liquid phases can be separated in a settler after cooling asdesired and essentially sulfur-free dimethyl formamide is withdrawn fromthis settler and recycled to the dimethyl formamide absorbing unit.Liquid sulfur is also withdrawn from this settler as the heavy liquidphase.

The gas that is deacetylenized and simultaneously desulfurized in thedimethyl formamide absorber generally comprises major portions ofmethane, hydrogen, ethylene and other olefins being the components ofinterest, and containing contaminants such as acetylene, methylacetylene and one or more sulfur compounds such as H₂ S, COS and/or CS₂.The gas usually further contains carbon oxides and some free oxygen;alternatively a small amount of oxygen can be added to provide same,e.g., by adding some air. The exact composition of the gas treated inaccordance with this invention is not critical except for the presenceof acetylenic compounds and oxidizable sulfur compounds as contaminantsto compounds of economic significance.

The preferred gas treated in accordance with this invention is coke ovengas. This gas is produced by the old and well-known process of coking orcarbonizing coals wherein considerable volumes of gas are generated bothby decomposition and partial oxidation. Processes and gases generatedtherefrom are widely described such as in the Encyclopedia of ChemicalTechnology, Kirk-Othmer, Volume 4, pages 400-423, particularly 415-416.

In accordance with another aspect of this invention, we have discoveredthat entrained dimethyl formamide, utilized as an absorption solution inan acetylene gas absorption or treating process, can be readilyrecovered from absorber overhead deacetylenized gas if a liquid streamselected from the group consisting of paraffinic hydrocarbons having 4-8carbon atoms per molecule containing at least one tertiary carbon atom,and aromatic hydrocarbons having 6-10 carbon atoms per molecule, isinjected into the absorber above the location of injection of the liquiddimethyl formamide. A particularly preferred variation of thisembodiment of the invention consists in utilizing the above-describedabsorption process in combination with an alkylation process. In thiscombined process, a gas stream containing at least one olefin and atleast one acetylene is contacted with dimethyl formamide. The stream ofnon-absorbed gas comprising olefin (alkylatable material) is utilized inan alkylation step, together with an alkylant (alkylating agent) such asto produce an alkylate. A portion of the alkylant is also injected intothe dimethyl formamide absorber above the location of injection ofliquid dimethyl formamide such as to "wash down" the dimethyl formamidethat may be vaporized and/or entrained into the overheadolefin-containing gas stream. The alkylant that is contained in theoverhead gas stream leaving the dimethyl formamide absorber does nothave to be separated from this gas stream because this alkylant is alsoa feed material to the downstream alkylation reaction. In thisalkylation reaction the olefin and the alkylant are reacted to form analkylate in the presence of a catalyst. Alkylation processes as such arewell known in the art. Examples for such alkylation processes are thealkylation of ethylene, propylene and/or butylene with isobutane orisopentane in the presence of, e.g., an HF catalyst, and the alkylationof ethylene with benzene in the presence of an aluminum halide orhydrofluoric acid catalyst.

The olefin plus isoparaffin alkylation process results in high-octanealkylates useful as premium motor fuel components. The ethylene plusaromatics alkylation process results in such products as ethylbenzene(useful as an intermediate to make styrene-polystyrene), cumene (usefulas a fuel additive), and diethylbenzene (useful as an intermediate tomake divinylbenzene and polymers incorporating it). In thefirst-mentioned alkylation process, the isoparaffin is injected to washdown the dimethyl formamide whereas the aromatic hydrocarbon is used forthis purpose if the purified olefin stream is used in the second type ofalkylation (with an aromatic hydrocarbon).

In the preferred embodiment of this invention in which coke oven gas isdeacetylenized by contacting the coke oven gas with dimethyl formamide,the liquid hydrocarbon that is injected above the location of dimethylformamide injection into the absorber is benzene. The overhead gasleaving the dimethyl formamide absorber contains ethylene and benzene,and this gas is passed to an alkylation unit in which ethylbenzene isproduced. Processes for converting ethylene and benzene to ethylbenzeneare described in more detail, e.g., in the U.S. Pat. Nos. 3,123,650;2,372,320 and 2,456,435.

The most preferred embodiment of this invention combines both thedesulfurization, the acetylene removal from the gas and the dimethylformamide removal from the raffinate gas as described above. Thus inaccordance with the most preferred embodiment of this invention, a gascomprising an olefin, oxygen, and as contaminants, acetylenes and one ormore oxidizable sulfur compounds, e.g., those selected from the groupconsisting of H₂ S, COS, CS₂, is contacted in an absorber with liquiddimethyl formamide, such as to remove the acetylenes from this gas andat the same time to convert the sulfur compounds into elemental sulfur.Above the locus of injection of the liquid dimethyl formamide, ahydrocarbon liquid stream, as defined above, is injected such as toremove substantially all of the dimethyl formamide contained in thedeacetylenized and desulfurized gas stream. The liquid bottoms effluentfrom the DMF absorber is separated into a gas stream comprisingacetylene, a first liquid stream consisting of essentially pure dimethylformamide and a second liquid stream consisting essentially of sulfur.The gaseous overhead stream leaving the absorber comprises the olefin aswell as a portion of the liquid hydrocarbon stream injected. This liquidhydrocarbon stream, in accordance with this preferred embodiment of theinvention, constitutes at least a portion of the alkylant with which theolefin is reacted in a downstream alkylation process.

Further details and preferred embodiments will become apparent to thoseskilled in the art from the following description of the drawing, whichshows a schematic diagram for the removal of impurities by absorption indimethyl formamide from coke oven gas.

Into a dimethyl formamide absorber 1, a coke oven gas stream isintroduced via line 2. This coke oven gas stream is contacted withliquid dimethyl formamide introduced via line 3 into the upper portionof the dimethyl formamide absorber 1. Above the locus of injection ofthe dimethyl formamide into the absorber 1, a stream of liquid benzeneis introduced via line 4. This stream of liquid benzene effectivelywashes out all of the entrained and/or vaporized dimethyl formamidecontained in the non-absorbed upflowing gas stream, returning it to thebulk of the dimethyl formamide in liquid form. The non-absorbed gaseousstream, having the acetylenes and entrained dimethyl formamide removed,leaves the dimethyl formamide absorber 1 via line 5 overhead. Thisstream is commonly known as residue gas, but is more properly referredto here as the purified olefin-containing stream. A portion of thebenzene is vaporized into this stream, and it may be said that thisbenzene takes the place of the vaporized dimethyl formamide by virtue ofthe relative vapor pressures. Within absorber 1, in contact with liquiddimethyl formamide, low concentrations of sulfur compounds aresubstantially oxidized by the low concentration of oxygen in the cokeoven gas feed to form elemental sulfur and water from H₂ S (or carbondioxide from COS and CS₂), the sulfur appearing as a highly dispersedsecond phase in the dimethyl formamide.

The liquid effluent leaving the dimethyl formamide absorber 1 via line 6as bottoms-enriched solvent consists essentially of two phases, namelythe dimethyl formamide having dissolved therein the extracted acetylenesand sulfur present either as liquid or dispersed solids, depending upontemperature. This stream is passed via an indirect heat exchanger 7 to aflash tank 8. From this flash tank 8, a liquid stream 28 stillcontaining the two phases of dimethyl formamide and sulfur is passed viaa heat exchanger 9 to a stripper 10. From this stripper 10, equippedwith a reboiler 11 as well as a reflux unit 12, a gaseous streamconsisting essentially of acetylenes is withdrawn via line 13. Thereflux unit 12 consists of a cooler-condenser 14, an accumulator vessel15 and a reflux pump 16.

The liquid effluent from the stripper 10 is passed via line 17 throughthe indirect heat exchanger 9 and via line 18 to a hot liquid phaseseparation settler 19. From this settler 19 liquid sulfur is withdrawnvia line 20 and liquid dimethyl formamide is withdrawn via line 21. Thisliquid dimethyl formamide is pumped by pump 22 through the heatexchanger 7 and a further cooler 23 to line 3 for reinjection into thedimethyl formamide absorber 1. Makeup dimethyl formamide to compensatefor losses in the system and withdrawal for purification purposes (notshown) is added via line 24.

The gaseous effluent from the flash drum 8 is passed via line 25 andcompressor 26 to line 27 via which this compressed gas is reinjectedinto the dimethyl formamide absorber 1 near the bottom thereof toincrease the degree of acetylenes absorption and ethylene rejection bythe DMF in absorber 1.

In the following, a typical calculated material balance is given for thesystem shown in the drawing. The quantities shown in this followingtable are kg/hr and the stream numbers are the same as those used in thedrawing.

                                      TABLE I                                     __________________________________________________________________________    Material Balance Calculated for a Dimethyl Formamide                          Absorption System as Shown in the Drawing                                     Stream                                                                        No.   2   4   3   5   28  13  17  20  24                                      __________________________________________________________________________    Component:                                                                    H.sub.2                                                                             739         739                                                         N.sub.2                                                                             1464        1464                                                        O.sub.2                                                                             319         237                                                         CO    1185        1185                                                        CO.sub.2                                                                            5338        4965                                                                              399 399                                                 COS   73          --  38  38                                                  H.sub.2 S                                                                           169         --  15  15                                                  CH.sub.4                                                                            4243        4243                                                                              --  --                                                  C.sub.2 H.sub.2                                                                     194         --  194 194                                                 C.sub.2 H.sub.4                                                                     6345        6250                                                                              95  95                                                  C.sub.2 H.sub.6                                                                     2241        2241                                                                              --  --                                                  C.sub.3 H.sub.4                                                                     149         --  149 149                                                 C.sub.3 H.sub.6                                                                     575         480 95  95                                                  C.sub.3 H.sub.8                                                                     274         274 --  --                                                  C.sub.4 +                                                                           1745        --  1745                                                                              1745                                                C.sub.6 H.sub.6                                                                     --  84      42  42  42                                                  DMF   --      35860                                                                             --  35860                                                                             20  35840   20                                      H.sub.2 O                                                                           --          --  82  82                                                  S     --          --  163 --  163 163                                         TOTAL 25053                                                                             84  35860                                                                             22120                                                                             38877                                                                             2874                                                                              36003                                                                             163 20                                      __________________________________________________________________________

The operating conditions for the various units are exemplified in thefollowing table by the temperatures and pressures of the streams, thecompositions of which have just been shown:

                                      TABLE II                                    __________________________________________________________________________    Operating Conditions                                                          Stream                                                                        No.   2   4   3   5   28  13  17  20  24                                      __________________________________________________________________________    Temp., ° C                                                                   27  16  16  16  90  38  181 135 16                                      Pressure,                                                                           2068                                                                              2068                                                                              2068                                                                              1993                                                                              1103                                                                              172 220 172 2068                                    kPa, abs.                                                                     __________________________________________________________________________

Reasonable variations and modifications which will become apparent tothose skilled in the art can be made in the present invention withoutdeparting from the spirit and scope thereof.

We claim:
 1. A process for the removal of acetylenes fromacetylene-containing gases comprising:(a) extracting a gas containing anolefin selected from the group consisting of ethylene, propylene,butylene, and mixtures thereof and acetylenes with liquid dimethylformamide in an extraction zone, (b) injecting a liquid hydrocarbonselected from the group consisting of paraffinic hydrocarbons having 4-8carbon atoms per molecule and containing at least one tertiary carbonatom per molecule and aromatic hydrocarbons having 6-10 carbon atoms permolecule into said extraction zone at a location above the feed inletfor the dimethyl formamide, and (c) withdrawing a gaseous effluentstream from said extraction zone comprising said olefin and saidhydrocarbon and being essentially acetylene-free and introducing atleast a portion of said gaseous effluent into an alkylation zone andreacting said olefin and said hydrocarbon in the presence of analkylation catalyst to form an alkylate in said alkylation zone.
 2. Aprocess in accordance with claim 1 wherein said liquid hydrocarbon isbenzene and wherein said olefin is selected from the group consisting ofethylene, propylene and mixtures thereof.
 3. A process in accordancewith claim 2 wherein said alkylation catalyst is selected from the groupconsisting of aluminum halide based alkylation catalysts and HF-basedalkylation catalysts.
 4. A process in accordance with claim 1 whereinsaid gas is a coke oven gas consisting essentially of hydrogen,nitrogen, oxygen, carbon monoxide, carbon dioxide, COS, H₂ S, methane,acetylene, ethylene, ethane, methylacetylene, propylene, propane andsome heavier hydrocarbons.